7-azaindole dimer Takeuchi and Tahara, Chem. Phys. Lett. 277, 340 - - PowerPoint PPT Presentation

Excited-state reaction dynamics in non-polar solvents: Excited-state double proton transfer of 7-azaindole dimer

• J. Phys. Chem. A 103, 4808 (1998)

PNAS 104, 5285 (2007)

• Chem. Phys. Lett. 347, 108 (2001)

Takeuchi and Tahara, Chem. Phys. Lett. 277, 340 (1997)

7-Azaindole Dimer

The Most Famous Model System of the Base Pair

Excited-State Double Proton Transfer of 7-Azaindole Dimer

Absorption and Steady-State Fluorescence Spectra of 7-Azaindole

270 nm excitation; 1x 10-2 M in hexane

0.2 ps 1.1 ps

Takeuchi and Tahara, J. Phys. Chem. A, 102, 7740 (1998).

Femtosecond time-resolved fluorescence of 7-Azaindole dimer

Quantitative Analysis of Time-Resolved Fluorescence





r i i

k d k   ) ( ) (

i

k





i i i

f d k    ) ( 1

2

: fluorescence spectrum in -space : radiative decay rate (inverse of radiative lifetime) : oscillator strength

) ( ) (

2

 

i i

k v k  

( )

Fluorescence Spectrum of Each Component





i i i

f d k    ) ( 1

2

• scillator strength

Properties of the Three Fluorescence Components

Fluorescence Anisotropy

Anisotropy Change of Fluorescence from 7-Azaindole Dimer

270 nm excitation; 1x 10-2 M in hexane

1Lb and 1La States of 7-Azaindole

It is well known that 7-azaindole (monomer) has two low-lying excited states, 1La and 1Lb, which are located closely in energy.

Fluorescence Spectra Oscillator Strength Fluorescence Anisotropy

Takeuchi and Tahara, J. Phys. Chem. A 102, 7740 (1998). Takeuchi and Tahara, Chem. Phys. Lett. 277, 340 (1997).

Mechanism of Excited-State Double Proton Transfer

• A. Douhal, S. K. Kim & A. H. Zewail, Nature, 378, 260 (1995)

“We find that the first step occurs on a timescale a few hundred femtoseconds, whereas the second step, to form the full tautomer, is much slower, taking place within several picoseconds.”

Femtosecond time-resolved mass spectroscopy in the gas phase (supersonic jet).

• A. W. Castleman Jr.

(Penn State Univ.)

• Chem. Phys. Lett. 287, 1 (1998)

Zewail did the experiments also in solution！

• J. Chem. Phys. A 103, 7419 (1999)
• J. Chem. Phys. A 102, 669 (1998)

Fluorescence Spectra Oscillator Strength Fluorescence Anisotropy

Deuterium Effect

Mechanism of Excited-State Double Proton Transfer

Michael Kasha

PNAS 96, 8338 (1999)

Nobody can be sure what they observed because ion reactions can occur with ionization or Coulomb explosion!

Michael Kasha

PNAS 96, 8338 (1999)

For solution, the conclusion of Takeuchi & Tahara is much more trustable!

Michael Kasha

PNAS 96, 8338 (1999)

Castleman Jr.

NAS Member from 1998 NAS Member from 1971

Drastic increase of the number of related paper

Web of Science

Search by “7-azaindole dimer” & “double proton

Year Tahara’s J. Phys. Chem. A (1998) Tahara’s Phys. Lett. (1997) Zewail’s Nature (1995) Kasha’s PNAS (1999) Castleman Jr.’s PNAS (1999) Zewail’s J. Phys. Chem. A (1998) Zewail’s J. Phys. Chem. A (1999)

Fluorescence Spectra Oscillator Strength Fluorescence Anisotropy

Deuterium Effect

Mechanism of Excited-State Double Proton Transfer

Excitation Wavelength Dependence

Absorption Spectrum of 7-Azaindole Dimer and Excitation Wavelengths

Absorption Spectrum of Dimer (obtained from spectral decomposition)

Excitation-wavelength dependence of the dimer fluorescence decay

Detection Wavelength 380 nm

1x 10-2 M in hexane

Femtosecond fluorescence from 7-azaindole dimer (Red-edge excitation)

single exponential!

Excitation-Wavelength Dependence

• f the Fluorescence Anisotropy

Fluorescence wavelength 380 nm

Transition Dipole Moment Directions of 7-Azaindole Dimers

The absolute direction of transition moment is taken from the value determined for the monomer in the supersonic jet.

• A. Nakajima et al.

Excited-State Double Proton Transfer of 7-Azaindole Dimer

“The answer to concerted versus step-wise controversy for the double proton transfer mechanism of 7-azaindole dimer in solution”

Takeuchi and Tahara, Proc. Natl. Acad. Sci. USA, 104, 5285 (2007)

It was NOT the end！

Kwon and Zewail, PNAS 104, 8703(2007).

• S. Take

Takeuch uchi i an and d T. Tah ahara, ara, PNAS NAS 104 104, 5285 (2 (2007)

Edit ited ed by N. . J. . Turro Approved ved January ry 12, , 2007

Oh Oh-H H Kwo won n an and d A. H.

• H. Zewa

wail, PNAS NAS 104 104, 8703 (2 (2007)

Co Contri ributed buted by A. . H.

• H. Zew

ewail ail April il 3, 20 , 2007

J.

• J. Cat

atalan, alan, PNAS NAS 105 105, E78 (2 (2008) Com

• mme

ment nt Oh Oh-H Kwo won an and d A. H. Zewa wail, PNAS NAS 105 105, E79 (2 (2008) Re Repl ply

Oh-H Kwon and A. H. Zewail, PNAS 105, E79 (2008) As we pointed out in our recent publication, another group of researchers stated, “The concerted mechanism, by its definition, does not require such a strict simultaneity, but it

• nly means that the motions of the two

protons are correlated”(6). It seems to us that the initial strict definition of concertedness (with the C2h symmetry path) is softened toward a breakage of C2h symmetry and asynchronous motion. At the end, it seems not profitable to have in the scientific literature the same claims in different colors, and we hope that this letter will be the epilogue.

Reference 6. Takeuchi S, Tahara T (2007), Proc Natl Acad Sci USA 104:5285-5290.

This was the end！